This invention relates generally to an electronic ballast and, more particularly, to a scheme for identifying the type of fluorescent lamp which is being powered by the electronic ballast.
There are many different types of fluorescent lamps including preheat and rapid start. Not only do each of these lamp types have different ratings for ignition and/or steady state operation, but within each lamp type there are different ratings for ignition and/or steady state operation. These differences can be expressed, in part, by voltage-current (V-I) characteristic curves. A ballast inverter should be driven based on the V-I characteristic curve of the lamp.
A typical ballast is designed to supply a specific starting voltage and load current based on the V-I characteristic curve of the lamp to be powered by the ballast. Different ballasts are therefore required based on the lamp load to be powered. No one ballast can be used for all these different types of lamps. With the increasing number of lamps available, more and more different types of ballasts are required. Many of these lamps are produced in relatively small numbers, making the manufacturing cost for the associated ballast relatively high. Ballasts designs are further complicated by the number of different ballasts designs required.
One approach which has been proposed in attempting to solve the foregoing problems, as disclosed in U.S. Pat. No. 5,039,921, identifies the lamp to be powered based on the lamp's starting voltage. Three different types of V-I characteristic curves are stored in and can be accessed from a memory based on the lamp's starting voltage. The accessed V-I characteristic curve is used in driving the ballast inverter. Unfortunately, many lamps have the same or about the same starting voltage and therefore cannot be distinguished from each other based on the starting voltage. The starting voltage also changes during the lifetime of the lamp thereby complicating recognition of the lamp based on starting voltage.
It is therefore desirable to provide an improved electronic ballast which can power a number of different types of lamp loads. The improved ballast should be able to distinguish among a number of different lamp loads having the same starting voltage.